In the case of relatively large photovoltaic installations, in particular open-field plants, also called solar parks, the electrical power which is produced is generally fed directly into a medium-voltage power supply grid, for example a 20 kilovolt (kV) power supply grid. Such open-field plants generally have a multiplicity of photovoltaic modules, a plurality of which are connected in series to form so-called strings. A plurality of the strings are frequently connected together in order to supply the power produced by them in the form of direct current to one of possibly a plurality of inverters that are provided. On the output side, inverters are connected to a primary winding of a medium-voltage transformer. In this case, one medium-voltage transformer can be provided for each inverter, or a plurality of inverters can be connected to one medium-voltage transformer, possibly with separate primary windings. The power which is produced is fed, possibly by means of a coupling contactor, into the medium-voltage power supply grid by the output or outputs on the secondary side of the transformer.
A photovoltaic installation of this type, in which the inverters are connected directly to the medium-voltage transformers, is known from the article “Enel's 3-MW Power Station Preliminary Design”, C. Corvi et al., 10th European Photovoltaic Solar Energy Conference, 1991. In this case, inverters are used which have inverter bridges which are fitted with thyristors. The inverters are commutated by the power supply grid, that is to say they draw the switching voltage, as well as the commutation voltage for the thyristors, from the electrical power supply grid.
Because of their relatively low efficiency, modern inverters with thyristor-fitted inverter bridges are still used only rarely. It is more common to use switching transistors in the inverter bridges, in order to allow the inverter to be operated by pulse-width modulation. In this case, the transistors used are generally IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors). For protection against high voltages in the reverse-biased direction, these transistors are normally protected by a diode which is connected anti-parallel wise and which is forward-biased when the transistor is reverse-biased, and which is frequently already integrated in the transistor. These diodes, which are also referred to as freewheeling diodes, form a full-wave rectifier from the power supply grid connection to the DC (direct-current) voltage intermediate circuit of the inverter. When an AC (alternating-current) voltage is present on the power supply grid side of the inverter, but the photovoltaic modules are not producing an adequate voltage, a reverse current flow occurs by the freewheeling diodes through the photovoltaic modules, with power being consumed from the AC voltage power supply grid.
In order to prevent such a reverse current flow, it is known for the inverters each to be connected to the medium-voltage transformers by an AC low-voltage contactor. The AC low-voltage contactors are used to disconnect the inverter from the power supply grid when there is no incident solar radiation, or when this is inadequate, thus preventing a reverse current flow through the photovoltaic modules, with power being consumed from the power supply grid.
In addition, the AC low-voltage contactors can be used to decouple the respective inverter selectively from the medium-voltage transformer in the event of an overcurrent or short circuit, if the required power supply grid parameters are infringed (voltage, frequency, reactive power feeding, etc.), or if an inverter fails. However, fitting every inverter with an associated AC low-voltage contactor and a monitoring device for compliance with the power supply grid feed parameters is complex and costly.
Furthermore, it is known to use an AC contactor located between a medium-voltage transformer and the power-supply grid for fault protection, as for example described in the article “Electrical fault protection for a large photovoltaic power plant inverter”, by Collier and Key, Photovoltaic Specialists Conference Record, IEEE, 1988. According to this document, the AC contactor is operated in case of a fault after DC connectors have decoupled the photovoltaic modules from the inverter.